With the development of more and more sophisticated electronic components, relatively extreme temperatures can be generated. This is clearly true with respect to electronic components capable of increasing processing speeds and higher frequencies, having smaller size and more complicated power requirements, those generating additional lighting advancements or exhibiting other technological advances. These components include microprocessors and integrated circuits in electronic and electrical components and systems as well as in other devices such as high power optical devices. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates these negative effects.
With the increased need for heat dissipation from electronic devices caused by these conditions, thermal management becomes an increasingly important element of the design of electronic products. As noted, both performance reliability and life expectancy of electronic equipment are inversely related to the component temperature of the equipment. For instance, a reduction in the operating temperature of a device such as a typical silicon semiconductor can correspond to an exponential increase in the reliability and life expectancy of the device. Therefore, to maximize the life-span and reliability of a component, controlling the device operating temperature within the limits set by the designers is of paramount importance.
One potential way to dissipate heat from an electronic component is by use of a flexible graphite thermal interface—that is, a thermal interface between the heat-generating component and another component such as a heat sink. Because of the anisotropic nature of flexible graphite sheet, it is uniquely effective at dissipating heat from a source, to effectively manage the heat generated in an electronic device or system. However, such a device prefers a low contact resistance between the various components of the system, such as the heat generating electronic component, the thermal interface material and the heat dissipating component.
Graphite is made of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphite consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphite has a high degree of preferred crystallite orientation. Graphite possesses anisotropic structures and thus exhibit or possess many properties such as thermal conductivity that are highly directional.
Briefly, graphite may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or lamina of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two sets of axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers (parallel to the planar direction of the crystal structure of the graphite) or the directions perpendicular to the “c” direction.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Graphite can be treated so that the spacing between the superposed carbon layers or lamina can be appreciably opened up so as to receive, or intercalate, other species between the carbon layers. Upon heating, the intercalated species decompose and volatilize to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction and thus form an expanded graphite structure (also referred to as exfoliated or intumesced graphite) in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is up to about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated articles and flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is up to about 80 or more times the original “c” direction dimension into integrated articles and flexible sheets by compression, without the use of any binding material, is believed to be possible due to the excellent mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the graphite material, as noted above, also possess a high degree of anisotropy with respect to thermal conductivity, comparable to the graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Generally, the process of producing flexible, binder-less anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is up to about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.02 g/cc to about 2.0 g/cc. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing with the force imparted by roll pressing of the sheet material. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces, comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces, comprising the “a” directions and the properties of the sheet are very different, by more than an order of magnitude, for the “c” and “a” directions. For instance, the thermal conductivity of flexible graphite sheet varies significantly between the “c” and the “a” directions (i.e., about 2-40 watts per meter-° C. (W/m° C.) vs. about 150-600 W/m° C.).
In the manufacturing process the electronic device is typically built from its case up. In other words, the case is provided first, and then the heat dissipating material is secured to the case. Thereafter, an interface material is applied to a surface of the dissipating material and next the electronic device is applied to the interface material and secured to the dissipating component.